1. Field of the Invention
The invention relates to a strong alkali borosilicate glass as well as to a method for the production thereof and the use thereof.
2. Description of Related Art
Alkali borosilicate glasses are known as such and, for example, are commercially available under the trade names Duran, Borofloat 33, or also Pyrex. Duran, for example, contains 80.2 wt % SiO2, 13.4 wt % B2O3, 2.3 wt % Al2O3, 3.5 wt % Na2O and 0.6 wt % K2O.
Alkali borosilicate glasses are characterized by a low sensitivity toward change in temperature as well as by a form stability up to high temperatures. Moreover, glasses of this type show an outstanding chemical resistance on contact with liquids. For this reason, these glasses are employed for a large number of applications as household and commercial glass. These kinds of glass find use in particular in laboratories, but also in the construction of industrial equipment. Thus, for example, glass pipelines of any length are used for wastewater gases or for the transport of aggressive chemicals. Another use is found in the pharmaceutical sector.
Alkali borosilicate glasses also find use in electrical engineering for electrical insulation.
Glass is also becoming increasingly widely used in building construction—for example, in the design of facades, for balustrade elements, for roofs, for doors, and also for partition walls. This entails an increased danger in the event that a fire breaks out. Conventional flat glasses burst even when exposed to heat on one side for a short time, as a result of which shards of large area fall out and enable the fire to spread into adjacent rooms. In order to prevent this, it was already attempted earlier to prevent the shattering of glass panes by insertion of a wire mesh, so that, even when a glass pane cracks, the shards thereof are held in place in the structure by the wire mesh.
Fire-resistant glasses that fulfill the provisions of fire resistance classes G and F (DIN 4102 Part 13 (ISO 834)) have also already been developed. This requires that the glazing, including the frames and mounts, prevent penetration of flames and combustion gas according to a standard temperature-time curve (UTTC) for at least 30 or 60, 90, or 120 minutes in order to be assigned to the classes G30, G60, G90, and G 120. Analogous provisions apply to the classes F30, F60, F90, and F120. Moreover, in the case of fire resistance class F, the glass may heat up by no more than 140° C. on average above the initial temperature on the side facing away from the fire.
Fire-resistant glazing made of prestressed alkali borosilicate glasses is also known and is commercially available for the fire resistance classes G and F. These glazings have a low coefficient of linear longitudinal thermal expansion of 3.3×10−6 K−1, for example. This low coefficient of linear longitudinal thermal expansion reduces the thermal stresses arising in the glass in the case of fire, so that such glasses make possible higher fire resistance times with, at the same time, a smaller edge distance or depth margin. However, these glasses exhibit the drawback that, on account of the low coefficient of linear longitudinal thermal expansion, only a very small prestress can be introduced into the glass on conventional air tempering apparatus, so that these glasses generally do not fulfill the desired requirements placed on safety glasses, such as, for example, the aforementioned DIN 1249.
For this reason, alkali borosilicate glasses that have a coefficient of thermal expansion of greater than 3.5·10−6 K−1 have already been developed. Glasses of this type are described in DE A 42 30 607, for example. A drawback is the required content of ZrO2 in these glasses. It has been found that, owing to non-fused residues of ZrO2, for example, these glasses exhibit a tendency toward spontaneous fractures during thermal prestressing or tempering.
Alkali borosilicate glasses for fire-resistant glazing, which fulfill the requirements placed on temperable fire-resistant safety glass, are also described in DE A 43 25 656. These glasses are also not free of drawbacks. It has been found in the case of these glasses that, during forming processes using the float process, for example, the Zn2+ present in the glass melt is reduced in the surface of the glass sheet to Zn0 owing to the strong reducing conditions (oxygen partial pressure p(O2) in the float bath of less than 10−10 bar). However, Zn0 can be readily volatilized, so that it sublimes out of the glass and condenses in the float bath on components, such as, for example, the assist rollers, which then have to be cleaned repeatedly during the process in a tedious operation. Moreover, non-sublimated, but nonetheless reduced Zn forms a coating on the surface of the flat glass, which greatly impairs the quality of the glass.
Moreover, this type of glass necessarily contains ZrO2, which, as stated above, exhibits the drawback in fire-resistant safety glasses that even small amounts of non-fused ZrO2/ZrSiO4 or secondarily crystallized ZrO2/ZrSiO4 lead to spontaneous fractures during tempering. Moreover, ZrO2 worsens the melting behavior, resulting in substantially higher energy costs.
DE A 195 15 608 describes an alkali borosilicate glass with a coefficient of linear longitudinal thermal expansion of 3.9-4.5·10−6 K−1. This glass is suited especially for fully electrical melting under cold top conditions. However, ZrO2 is also necessarily required as a component for this glass, so that the glass is not optimally suited as a fire-resistant safety glass for the reasons already mentioned.
Highly temperable glasses with an expansion in the range of 3.39·10−6 K−1 to 5.32·10−6 K−1 are also described in DE A 27 56 555. These glasses exhibit the drawback of an intentionally chosen, high coefficient of thermal expansion above the transformation temperature. This leads to problems in the dimensional stability in the event of thermal stress during, for example, a coating process at temperatures below but near the transformation temperature.
Glasses with a high chemical resistance are described in DE A 44 30 710. However, these glasses are difficult to melt and exhibit a high density.
Described in EP A 0 576 362 is a thermally stable and chemically resistant alkali borosilicate glass for fire-resistant glazing. However, this glass exhibits the drawback that, because of the high transformation temperature of >600° C., it cannot be tempered on conventional air tempering apparatus and, because of the low viscosity in the range of the melting temperatures (log (η/cP)=2 at approximately 1,450° C.), many refining agents, such as, for example, NaCl and KCl, cannot be used.
This also applies to JP A 61 024 344, in which the melting temperature is also too low for sodium chloride refinement. Moreover, the proportion of V2O5 that is necessarily required for forming in the float process is detrimental, because, in this case, the V5+ ion is reduced in the float bath. Moreover, the V2O5 has a very detrimental effect also on the very high light transmittance being sought.
Finally, an alkali borosilicate glass with good fire and thermoshock resistance is described in U.S. Pat. No. 5,776,844. In this case, however, the coefficient of linear thermal expansion lies between 8.0·10−6 K−1 and 9.5·10−6 K−1 and the strain point lies at only approximately 25 K above that of normal soda-lime glass. In addition, the glasses have a low content of SiO2. Glasses of this type often show a relatively low chemical resistance in comparison to alkali borosilicate glasses with a higher content of SiO2.
DE A 196 43 870 describes alkali borosilicate glasses that are chemically prestressed by ion exchange. The possible field of application of induction cooktops made of specialty glass is described in the specifications JP 2003 086337 A, JP 2003 217811 A, WO 2003/098115 A1, DE 102 43 500 A1, DE 101 22 718 C2, DE 101 50 884 A1, DE 103 551 60, GB 2 079 119 A, U.S. Pat. No. 6,051,821 A, WO 2004/018944 A1, and WO 2012/146860 A1.
Further specifications within the compass of the invention are: DE 37 22 130 A1, DE 40 122 88 C1, EP 0 588 000 B1, WO 96/33954 A2, JP 83145637 A, JP 89093437 A, SU 1284959 A, DE 44 28 235 C1, DE 1496637 A, FR 2389582 A1, JP 82160938 A, DE 588643 A, DE 2413552 A1, EP 1314704 B1, and WO 2012/146860 A1.